Endangered Sumatran tiger triplets have been born at London zoo to the mother whose cub drowned in the multimillion pound tiger enclosure last year.

The triplets were born to five-year-old Sumatran tigress Melati on 2 February. The first cub arrived at 12.28, the second at 12.59, and the last at 1.18, with keepers monitoring the birth using remote camera technology. It is not yet known what sex the cubs are.

Melati and her cubs, pictured, remain inside the den away from the public, with the mother occasionally venturing out for food. The father, six-year-old Jae Jae, remains in the main tiger enclosure, away from the cubs.

Zookeeper Teague Stubbington said: "We couldn't be more delighted with our new arrivals, and with how Melati is responding to her three cubs … we've even been able to observe key milestones like their eyes opening and their tentative first steps.

"While we still don't know whether they're boys or girls, we're starting to see their personalities develop. We've nicknamed one Trouble, as it's much bolder than the others – it was the first to start exploring its den and we've spotted it waking up its siblings when they fall asleep."

With Sumatran tiger numbers estimated to be as low as 300 individuals in the wild, the zoo says the birth of the cubs represents a huge achievement, "not just for ZSL London zoo but for the global breeding programme of this critically endangered species".

Three of the world’s rarest tigers have been born at ZSL London zoo, with zookeepers capturing their first moments on hidden cameras. Photograph: ZSL

In October last year, a newborn cub – the first to be born at the zoo in 17 years – drowned in the pool in the tiger enclosure. Only two weeks old, it had not yet been named or sexed.

It is thought that the mother, Melati, carried the cub outside but keepers still remain unclear as to how the cub got into the pool as there were no cameras in the wider enclosure.

A spokeswoman for the zoo said that with the first cub, keepers had been reticent to interfere given Melati's nervous disposition, but that changes had been made during the pregnancy to prevent a repeat of the situation.

"We've drained the pond and built a cub conservancy so that Melati has space to go outside with the cubs without going too far. The keepers are monitoring the situation 24/7 and taking turns to watch the cameras overnight."

London zoo's £3.6m Tiger Territory opened in March 2013 and was designed to encourage breeding of the endangered subspecies of tiger.

The as-yet unsexed triplets were born to five-year-old Sumatran tigress Melati, on 3 February 2014, after a 106-day pregnancy. Photograph: ZSL

The Sumatran tiger, whose habitat is the forests and jungles of Sumatra, Indonesia, is threatened by poaching, habitat loss and human conflict.

In the 1970s, there were estimated to be 1,000 Sumatran tigers in the wild, but current figures indicate that there are just 300. Should their numbers continue to decline at this rate, conservationists warn that Sumatran tigers will face the very real threat of extinction within the next decade.